In recent years, it has been more and more desired to output an image at a higher speed in the field of printers and copying apparatuses using an electrophotographic method. In order to increase the process speed of image formation, it is necessary to fully electrically charge a toner to stably develop an electrostatic latent image. In order to fully charge a toner, the friction charging property of a resin component of a toner have to be used. However, only by this method, since the amount of charge of a toner cannot be fully obtained, the initial rise of the density of an image becomes late, whereby a sufficient image density may not be obtained.
The needs for a toner which enables low-temperature fixing, so called a toner having a low-temperature fixing property, have become high not only from the view point of energy saving, since, when the process speed of image formation is increased, fixability is not securable if melting of the toner with a small quantity of heat is possible.
In Patent Documents 1, disclosed have been toner particles containing a polymer obtained by polymerizing at least a polymerizable monomer having a functional group selected from a group of a sulfonic acid group, a sulfonic salt group, a sulfonic acid ester group, and other polymerizale vinyl polymer, as a technology to prevent contamination of each member of a copying device and to obtain sufficient fixablity while stabilizing the charge property of the toner. However, the present inventors have revealed from our examination that, although certain effects can be recognized with the toner, further improvements are desired with respect to the low-temperature fixablility, the image density, and glossiness of the image.
On the other hand, in order to enable fixing at a lower temperature, use of a binder resin having a high sharp melt property in a toner has been known as one of the effective methods, and a polyester resin has been known to be excellent to have such a property (for example, refer to Patent Documents 2).
Accordingly, it is motivated to introduce a polymerizable monomer having a functional group selected from a group of a sulfonic acid group, a sulfonic salt group, a sulfonic acid ester group into a polyester resin (for example, refer to Patent Documents 3 and Patent Documents 4). However, the present inventors have revealed from our examination that, in the conventional method, it has been necessary to copolymerize such a polymerizable monomer with styrene and an acrylic ester, and then to mix in the toner in the manufacturing process of the toner particles, which cause notably low productivity.
On the other hand, a technique to hybridize a polyester resin and a radically polymerizable monomer to achieve sufficient elasticity at higher temperatures has been disclosed (for example, refer to Patent Document 5).
Patent Documents 1: Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2009-168963
Patent Documents 2: JP-A No. 5-027478
Patent Documents 3: JP-A No. 2002-351147
Patent Documents 4: JP-A No. 2010-271715
Patent Documents 5: JP-A No. 2011-095736